An extruded polystyrene resin foam has excellent heat insulating property and mechanical strength, thus a molded article is widely used as a heat insulator and the like. Therefore, styrenic resin foams in the form of board having a certain width have been widely used as insulating materials. Such a foam is usually produced as follows: a styrenic resin is heated and molten in an extruder, the melt is mixed with a physical blowing agent to make a foamable molten resin composition, the foamable molten resin composition is extruded and foamed through, for example, a slit die provided at the tip of the extruder into a low-pressure region, and, as necessary, the foam is molded by a shaping device connected to the outlet of the die.
As physical blowing agents used for producing extruded polystyrene resin foam, chlorofluorocarbons (hereinafter abbreviated as “CFCs”) such as dichlorodifluoromethane have been widely used. However, since CFCs have high risk of depleting the ozone layer, hydrogenated chlorofluorocarbons (herein after, abbreviated as “HCFCs”) having low ozone depletion potentials are used instead of CFCs. Nevertheless, since the ozone depletion potential of HCFCs are not 0 (zero), HCFCs still have risk of depleting the ozone layer. Under these circumstances, recently, hydrofluorocarbons (herein after, abbreviated as “HFCs”) having an ozone depletion potential of 0 (zero) and no chlorine atom in the molecule have been used as blowing agents.
HFCs are preferred from the view point of ozone depleting potentials, but HFCs have high global warming potentials, so that is still room for an improvement from the viewpoint of the global environmental protection. Therefor, methods of producing an extruded thermoplastic resin foams which have the ozone depletion potential of 0 (zero), also have low global warming potentials, and uses a blowing agents with small effect on the environment, have been studied.
As a physical blowing agent, alipatic hydrocarbons or an alicyclic hydrocarbons such as propane, normal butane, isobutane, normal pentane, cyclopentane, and isopentane (herein below, these hydrocarbons are referred to as “HC”) has an ozone depletion potential of 0 (zero) and low global warming potentials, and therefore there are a favorable blowing agents from the viewpoint of global environmental protection. In practice, some HCs are used as substitute blowing agents for the freons described above.
Meanwhile, the thermal conductivity of the extruded thermoplastic resin foam is also influenced by a blowing agent remaining in the foam. The permeation rates of the HCs through out a styrene resin are much lower than that of air, but higher than that of freons. As such, the HCs permeate a foam faster than the freons. Thus, when the HCs are used as a blowing agent, the thermal conductivity of a foam increases more quickly as compared with the thermal conductivity when freons are used as a blowing agent.
Therefore, in regard to the extruded thermoplastic resin foam that is obtained by using the HC as a physical blowing agent, it is difficult to maintain the heat insulating property for a long period of time.
A foam prepared by a resin used of a high gas barrier resin dispersed in a polystyrene resin has been suggested. Japanese Patent Application Laid-Open (JP-A) Nos. 2006-131719 and 2006-131757 disclose methods for producing an extruded thermoplastic resin foam which is produced by adding a nitrile resin or a vinyl alcohol resin to a polystyrene resin, and by using a blowing agent containing isobutane. JP-A-2006-131719 and 2006-131757 are the technology of maintaining the heat insulating property of a heat insulation foam board, and the technology of inhibiting the dissipation of a blowing agent having low thermal conductivity such as isobutane from the foam, by mixing a well-known gas barrier resin including a nitrile resin and a vinyl alcohol resin to a polystyrene resin blowing
Further, JP-A-2002-144497 discloses a technology of inhibiting the dissipation of a physical blowing agent from an extruded thermoplastic resin foam by coating the surface of the extruded thermoplastic resin foam with gas barrier coating film of a non-halogen within a month after manufacturing the extruded polystyrene resin foam. According to this method, however, there is a problem in that the heat insulating property cannot be maintained once the coating film is damaged during the cutting process of a heat insulation foam board or the installment of an extruded thermoplastic resin foam using such as nail, and thus practical value is not high because a special apparatus is required for the manufacture.
Meanwhile, JP-A-2000-136258 discloses a foam sheet obtained by extruding and foaming a mixture of polystyrene resin and polyester resin.
The foam sheet disclosed in JP-A-2000-136258 is a foam sheet for thermal molding which can be molded into a dish, a bowie, and the like. As such, thickness of the foam sheet is thin, and expansion ratio is small. In other words, the foam sheet is different from the foam of the present invention which is thick, board-like, and has high expansion ratio for the purpose of maintaining the heat insulating property for a long period of time. With regard to the foam sheet disclosed in JP-A-2000-136258, the reason for mixing a polyester resin with a polystyrene resin is only to improve the oil resistance of a polystyrene resin foam sheet. In addition, it is well known that an extruded heat insulation foam having a large thickness and high expansion ratio cannot be easily obtained by simple mixing of a polystyrene resin with a polyester resin.